“Removing the blanket ban on prisoners’ voting in elections, which was required by decisions of the European Court of Human Rights to comply with the Convention for the Protection of Human Rights and Fundamental Freedoms, involved a controversial area of social policy. It was for the Government to decide on the appropriate amending legislation to be enacted to comply with the law. The judiciary ought not to construe existing statutes so as to confer on the courts the function of deciding whether any given prisoner should lose his vote.”